In the typical gear production facility precision cutting of gear teeth is followed by heat treatment to harden the gears. The heat treatment often distorts the gears. Some types of gears are lapped as pairs after heat treatment to refine the tooth contact areas. Where high quality gears are necessary, final finishing is done after hardening by conventional grinding operations. American Gear Manufacturers Association Technical Paper No. 84FTM6, entitled "CBN Finish Grinding of Hardened Spiral Bevel and Hypoid Gears", by Gary J. Kimmet and published in 1985, discloses the use of cubic boron nitride (CBN) grinding for final finishing after hardening.
U.S. Pat. No. 4,761,867 of Vollmer et al., issued Aug. 9, 1988, discloses a method for producing ring gears in which a near net shape ring gear has its tooth surfaces cubic boron nitride ground after carburize heat treatment. This requires the grinding operation to be performed on a hardened surface and, while cubic boron nitride grinding reduces the potential for grinder burn, the problem still exists. Further, because this process requires metal removal after the heat treat operation, it is necessary to form a hardened depth that is thick enough to allow the required depth to remain after the grinding operation.
That patent also discloses as prior art the well known method of making a gear by cutting (i.e. machining) operations prior to heat treat. Typically the cutting operations include both rough and finish cuts; however the so-called finish cut introduces stresses which vary over the tooth surfaces. Those stresses contribute to heat treat distortions. In the prior art some gears are used "as is" after heat treatment, some are lapped, and some are ground.
The present invention is directed to overcoming one or more of the disadvantages described above.